1. Field of the Invention
The invention relates to the manufacture of sheet material used for packaging. More particularly it relates to the manufacture of bilaminar sheet material having raised pockets containing a gas, said pockets extending from either or both surfaces of the sheet.
2. Description of the Prior Art
In the past a variety of relatively complex apparatus, devices and methods have been suggested for producing sheet material of uniform thickness having closed pockets, blisters or bubbles. These prior art methods and apparatus are generally characterized by being intricate and complicated, with a resultant excessive cost for producing the finished product. A particular problem encountered in the many of these processes has been the lack of uniformity of thickness of the sheet material. This is due to the fact that when a tubular member is extruded through an annular circular slot and subsequently passed through the nip of two juxtaposed parallel cylindrical rollers the center portion of the flattened bilaminar sheet material, thus formed, is thinner than the edge portions. This thinning out of the central portion is due to internal stresses exerted upon the molten tubular member as it is drawn from a generally tube like member to a flattened member by the nip of the rollers. As would be evident the thinner center portion substantially weakens the sheet, as well as any raised pockets formed within said portion; the end result being a serious detraction from the cushioning qualities of the product.
The present invention has overcome many of the disadvantages of the prior art in that it provides for an apparatus and method for producing a bilaminar sheet material of substantially uniform thickness, having raised pockets thereon, which is relatively simple and inexpensive to operate.